1. Field
This document relates to a touch sensing apparatus and a driving method thereof.
2. Related Art
A user Interface (UI) enables communication between a person (user) and various electric and electronic devices to allow the user to easily control the devices. Examples of the UI include a keypad, a keyboard, a mouse, an on screen display (OSD), a remote controller having an infrared communication or radio frequency (RF) communication function, etc. User interface technology is developed to improve user's emotion and operation convenience. Recently, UI has been evolved to a touch UI, a voice recognition UI, a 3D UI, etc. The touch UI is basically installed in a mobile information device. To implement the touch UI, a touch screen is provided to a display device of home appliances or mobile information device.
Touch sensors that construct a touch screen can be implemented as capacitive touch sensors, resistive touch sensors, etc. A capacitive touch screen is applied to a wide range of applications because it has durability and definition higher than those of a resistive touch screen and can recognize multi-touch and proximity touch recognition.
A touch sensing apparatus including a display device and a touch screen determines whether contact (or proximity) of a conductive material is made and the contact point of the conductive material by sensing a touch sensor capacitance variation before and after touch (or proximity touch). The touch screen is generally located in or on a display panel of the display device, and thus noise is applied to voltages of touch sensors due to the influence of a driving signal of the display panel. This is because signal lines connected to the touch sensors and signal lines connected to pixels of the display panel electrically affect each other due to coupling thereof.
The noise is largely affected by a time variation in a data voltage applied to liquid crystal cells. The display panel includes a plurality of pixels as shown in FIG. 1. Each of the pixels has pixel capacitance including liquid crystal capacitance Clc and storage capacitance Cst. When the display panel is touched by a user's finger, the pixel capacitance can further include finger capacitance Cf. As the data voltage varies with time, a charging voltage of the pixel capacitance is changed to causes noise in the voltages of the touch sensors. In addition, the data voltage variation affects a first parasitic capacitance Cgd between a gate line and a data line, a second parasitic capacitance Cgp between the gate line and a pixel electrode, a third parasitic capacitance Cdp between the data line and the pixel electrode, and a fourth parasitic capacitance Cgc between the gate line and a common electrode, thereby bringing about noise in the voltages of the touch sensors.
Noise inflow due to the data voltage variation becomes a problem when one frame is time-divided into a display panel driving period T1 and a touch screen driving period T2, particularly, as shown in FIG. 2. When a difference between a data voltage for black gradation and a data voltage for white gradation is large, a variation in the charge quantity of a pixel (pixel capacitance and parasitic capacitance) increases when black gradation and white gradation are changed each other. Provided that the data voltage corresponding to white gradation is 5V and the data voltage corresponding to black gradation is 0V, the pixel charge quantity is varied by 5V (0V to 5V) in case of black-to-white gradation change. Considering inversion, the pixel charge quantity is varied by 10V (−5V to 5V) in the event of white-to-white gradation change. This pixel charge quantity variation is applied to the voltages of the touch sensors as noise to increase a variation (ΔX) in touch raw data. As a result, a touch sensor operates as if it senses a touch input even though the touch sensor is not touched.
FIG. 3 shows touch raw data when a screen displays black gradation and FIG. 4 shows touch raw data when the screen is divided into four areas which display black gradation and white gradation. When white gradation is displayed on the screen as shown in FIG. 4, touch raw data measured during the touch screen driving period (T2 of FIG. 2) is in the range of 60 to 125 irrespective of presence or absence of touch. This is a very high value compared to touch raw data corresponding to black gradation and may exceed a reference value for determining whether touch is applied or not. If touch raw data corresponding to a non-touched point exceeds the reference value, a touch recognition error is generated and sensitivity of the touch sensors is decreased.